Rainbow Electronics MAX1637 - Manual (Page 16)

MAX1637
Miniature, Low-Voltage,
Precision Step-Down Controller
16 ______________________________________________________________________________________
Use only specialized low-ESR capacitors intended for
switching-regulator applications, such as AVX TPS,
Sprague 595D, Sanyo OS-CON, or Nichicon PL series.
To ensure stability, the capacitor must meet both mini-
mum capacitance and maximum ESR values as given
in the following equations:
C OUT > V REF (1 + V OUT / V IN(MIN) ) / V OUT x R SENSE x
ƒ
R ESR < R SENSE x V OUT / V REF
where R ESR can be multiplied by 1.5, as discussed
below.
These equations are worst case, with 45 degrees of
phase margin to ensure jitter-free, fixed-frequency
operation, and provide a nicely damped output
response for zero to full-load step changes. Some cost-
conscious designers may wish to bend these rules with
less-expensive capacitors, particularly if the load lacks
large step changes. This practice is tolerable if some
bench testing over temperature is done to verify
acceptable noise and transient response.
No well-defined boundary exists between stable and
unstable operation. As phase margin is reduced, the first
symptom is timing jitter, which shows up as blurred edges
in the switching waveforms where the scope does not quite
sync up. Technically speaking, this jitter (usually harmless)
is unstable operation since the duty factor varies slightly.
As capacitors with higher ESRs are used, the jitter
becomes more pronounced, and the load-transient output
voltage waveform starts looking ragged at the edges.
Eventually, the load-transient waveform has enough ringing
on it that the peak noise levels exceed the allowable output
voltage tolerance. Note that even with zero phase margin
and gross instability, the output voltage seldom declines
beyond I
PEAK x R ESR (under constant loads).
Designers of RF communicators or other noise-sensi-
tive analog equipment should be conservative and stay
within the guidelines. Designers of notebook computers
and similar commercial-temperature-range digital sys-
tems can multiply the R
ESR value by a factor of 1.5
without affecting stability or transient response.
The output voltage ripple, which is usually dominated by
the filter capacitor’s ESR, can be approximated as
I
RIPPLE x R ESR . There is also a capacitive term, so the
full equation for ripple in continuous-conduction mode is
V
RIPPLE(p-p) = I RIPPLE x [R ESR + 1 / (2
πƒ
x C OUT )]. In
idle mode, the inductor current becomes discontinuous,
with high peaks and widely spaced pulses, so the noise
can actually be higher at light load (compared to full
load). In idle mode, calculate the output ripple as follows:
V RIPPLE(p-p) = (0.02 x R ESR / R SENSE ) + [0.0003 x L x
(1 / V OUT + 1 / (V IN - V OUT )) / R SENSE
2 x C F ]
Selecting Other Components
MOSFET Switches
The high-current N-channel MOSFETs must be logic-
level types with guaranteed on-resistance specifications
at V
GS = 4.5V. Lower gate-threshold specifications are
better (i.e., 2V max rather than 3V max). Drain-source
breakdown voltage ratings must at least equal the maxi-
mum input voltage, preferably with a 20% margin. The
best MOSFETs have the lowest on-resistance per
nanocoulomb of gate charge. Multiplying R
DS(ON) by
Qg provides a good figure of merit for comparing vari-
ous MOSFETs. Newer MOSFET process technologies
with dense cell structures generally perform best. The
internal gate drivers tolerate >100nC total gate charge,
but 70nC is a more practical upper limit to maintain best
switching times.
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I
2 R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. I 2 R losses are
distributed between Q1 and Q2 according to duty fac-
tor, as shown in the following equations. Generally,
switching losses affect only the upper MOSFET since
the Schottky rectifier usually clamps the switching node
before the synchronous rectifier turns on. Gate-charge
losses are dissipated by the driver and do not heat the
MOSFET. Calculate the temperature rise according to
package thermal-resistance specifications to ensure
that both MOSFETs are within their maximum junction
temperature at high ambient temperature. The worst-
case dissipation for the high-side MOSFET occurs at
both extremes of input voltage, and the worst-case dis-
sipation for the low-side MOSFET occurs at maximum
input voltage.
Duty = (V OUT + V Q2 ) / (V IN - V Q1 )
P D (UPPER FET) = I LOAD2 x R DS(ON) x duty + V IN x
I LOAD x
ƒ
x [(V IN x C RSS ) / I GATE + 20ns]
P D (LOWER FET) = I LOAD2 x R DS(ON) x (1 - duty)
where V Q = the on-state voltage drop (I LOAD x
R DS(ON) ), C RSS = the MOSFET reverse transfer capaci-
tance, I GATE = the DH driver peak output current capa-
bility (1A typ), and the DH driver inherent rise/fall time is
20ns. The MAX1637’s output undervoltage shutdown
function protects the synchronous rectifier under output
short-circuit conditions. To reduce EMI, add a 0.1µF
ceramic capacitor from the high-side switch drain to
the low-side switch source.